Lipids refer to substances that have a long fatty acid chain or a similar hydrocarbon chain in a molecule and are present in a living body or derived from a living thing. The lipids may be classified into storage lipids and structural lipids in membranes.
Storage lipids comprise C, H and O, and are generally soluble in organic solvent such as acetone. Triacylglycerol that is a storage lipid is present in a fat tissue of an animal body as energy storage. On the other hand, structural lipids in membranes such as phospholipids comprise a hydrophobic part (fatty acid part) and a hydrophilic part (phosphoric acid and base part) to exhibit amphipathic property. Generally, while storage lipids are soluble in acetone, structural lipids are insoluble in acetone. Such structural lipids are structural components of a biomembrane.
Structural lipids in membranes may be roughly classified into the following categories:                (1) Glycerophospholipids: examples include phosphatidylcholine (lecithin), phosphatidylethanolamine, etc.;        (2) Phosphosphingolipids: examples include sphingomyelin, ceramide ciliatine, etc.;        (3) Glycosphingolipids: examples include cerebroside, sulfatide, ganglioside, etc.; and        (4) Glycoglycerolipids: examples include galactosyldiacylglycerol existing in a microorganism or higher plant.        
The above (2) phosphosphingolipids and (3) glycosphingolipids are collectively called as sphingolipids.
Glycerophospholipids is a collective term for phospholipids having a glycerol in their backbone, examples of which include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, etc. Many of those glycerophospholipids are bound to a non-polar part (fatty acid part) at the sn-1 position of glycerol backbone by an ester bond (acyl bond), and some are bound at the sn-1 position by a vinyl ether bond (alkenyl bond) or an ether bond (alkyl bond). The former ones bound by a vinyl ether bond are also called as plasmalogens. Glycerophospholipids having a vinyl ether bond and those having an ether bond are collectively called as ether phospholipids.
While phospholipids are integral structural components of a biomembrane, approximately 18% of phospholipids of a mammalian biomembrane are plasmalogens that are vinyl ether phospholipids. In particular, many of plasmalogens are found in brain tissues, cardiac muscles, skeleton muscles, white blood cells and sperms. Many of plasmalogens are bound to polyunsaturated fatty acids such as eicosapentaenoic acid (EPA), docosahexaenoic acids (DHA), arachidonic acids (ARA), and the like at the sn-2 position of glycerol backbone. Therefore, they play not only a role as a reservoir of second messengers for signals between cells such as prostaglandin, leukotriene, etc., but also significant roles as cell fusion, ion transport, etc. In addition, since a vinyl ether bond (alkenyl bond) of plasmalogens is particularly susceptible to oxidative stress, they act as antioxidants at cell membranes.
In mammals, ether phospholipids having an alkyl bond are present although in a small amount. In particular, it is confirmed that phosphatidylcholine and phosphatidylethanolamine having an alkyl bond are present in a rat's brain hippocampus. Furthermore, it is known that ingested phospholipids having an ether bond (alkyl bond) are transformed into plasmalogens in vivo.
These days, it is reported, as shown in WO2011/083827 (Patent Document 1), that plasmalogens-type glycerophospholipids have an effect of brain neurogenesis. In addition, as shown in WO2012/039472 (Patent Document 2) and Ifuku et al., Journal of Neuroinflammation, 9:197 (2012) (Non-patent Document 1), it is reported that plasmalogens-type glycerophospholipids inhibit an increase of glia cells that is considered to be one of the causes triggering a central nervous system inflammation, thereby effective for decreasing central nervous system inflammation, and that they are particularly effective for preventing and treating neurodegenerative disease such as Alzheimer's disease.
It is reported, as shown in, for example, Patent Documents 1-7, that such plasmalogens-type glycerophospholipids may be obtained from a bird tissue such as chicken skin and chicken breast, in an easy manner on a massive scale. Furthermore, Patent Document 8 reports a method for producing ether phospholipids or plasmalogens-type glycerophospholipids from bivalve tissues, and demonstrates that the bivalve extracts produced by such a method are bound to a larger amount of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) at the sn-2 position of glycerol backbone compared to the conventional plasmalogens-type glycerophospholipids derived from chicken tissues.